Menu Close

The-flow-velocity-of-a-river-increases-linearly-with-the-distance-r-from-its-bank-and-has-its-maximum-value-v-0-in-the-middle-of-the-river-The-velocity-near-the-bank-is-zero-A-boat-which-can-move




Question Number 18269 by Tinkutara last updated on 17/Jul/17
The flow velocity of a river increases  linearly with the distance (r) from its  bank and has its maximum value v_0  in  the middle of the river. The velocity  near the bank is zero. A boat which can  move with speed u in still water moves  in the river in such a way that it is  always perpendicular to the flow of  current. Find  (i) The distance along the bank through  which boat is carried away by the flow  current, when the boat crosses the  river.  (ii) The equation of trajectory for the  coordinate system shown. Assume  that the swimmer starts from origin.
$$\mathrm{The}\:\mathrm{flow}\:\mathrm{velocity}\:\mathrm{of}\:\mathrm{a}\:\mathrm{river}\:\mathrm{increases} \\ $$$$\mathrm{linearly}\:\mathrm{with}\:\mathrm{the}\:\mathrm{distance}\:\left({r}\right)\:\mathrm{from}\:\mathrm{its} \\ $$$$\mathrm{bank}\:\mathrm{and}\:\mathrm{has}\:\mathrm{its}\:\mathrm{maximum}\:\mathrm{value}\:{v}_{\mathrm{0}} \:\mathrm{in} \\ $$$$\mathrm{the}\:\mathrm{middle}\:\mathrm{of}\:\mathrm{the}\:\mathrm{river}.\:\mathrm{The}\:\mathrm{velocity} \\ $$$$\mathrm{near}\:\mathrm{the}\:\mathrm{bank}\:\mathrm{is}\:\mathrm{zero}.\:\mathrm{A}\:\mathrm{boat}\:\mathrm{which}\:\mathrm{can} \\ $$$$\mathrm{move}\:\mathrm{with}\:\mathrm{speed}\:{u}\:\mathrm{in}\:\mathrm{still}\:\mathrm{water}\:\mathrm{moves} \\ $$$$\mathrm{in}\:\mathrm{the}\:\mathrm{river}\:\mathrm{in}\:\mathrm{such}\:\mathrm{a}\:\mathrm{way}\:\mathrm{that}\:\mathrm{it}\:\mathrm{is} \\ $$$$\mathrm{always}\:\mathrm{perpendicular}\:\mathrm{to}\:\mathrm{the}\:\mathrm{flow}\:\mathrm{of} \\ $$$$\mathrm{current}.\:\mathrm{Find} \\ $$$$\left(\mathrm{i}\right)\:\mathrm{The}\:\mathrm{distance}\:\mathrm{along}\:\mathrm{the}\:\mathrm{bank}\:\mathrm{through} \\ $$$$\mathrm{which}\:\mathrm{boat}\:\mathrm{is}\:\mathrm{carried}\:\mathrm{away}\:\mathrm{by}\:\mathrm{the}\:\mathrm{flow} \\ $$$$\mathrm{current},\:\mathrm{when}\:\mathrm{the}\:\mathrm{boat}\:\mathrm{crosses}\:\mathrm{the} \\ $$$$\mathrm{river}. \\ $$$$\left(\mathrm{ii}\right)\:\mathrm{The}\:\mathrm{equation}\:\mathrm{of}\:\mathrm{trajectory}\:\mathrm{for}\:\mathrm{the} \\ $$$$\mathrm{coordinate}\:\mathrm{system}\:\mathrm{shown}.\:\mathrm{Assume} \\ $$$$\mathrm{that}\:\mathrm{the}\:\mathrm{swimmer}\:\mathrm{starts}\:\mathrm{from}\:\mathrm{origin}. \\ $$
Commented by Tinkutara last updated on 17/Jul/17
Commented by mrW1 last updated on 19/Jul/17
u_y =u  y=ut  u_x =((2y)/d)v_0 =((2v_0 )/d)y with 0≤y≤(d/2)  u_x =((2(d−y))/d)v_0 =2v_0 (1−(y/d)) with (d/2)≤y≤d    u_x =(dx/dt)=(dx/dy)×(dy/dt)=u(dx/dy)  ⇒u(dx/dy)=((2v_0 )/d)y with 0≤y≤(d/2)  ⇒u(dx/dy)=2v_0 (1−(y/d)) with (d/2)≤y≤d  x=(v_0 /(ud))y^2 +C_1   C_1 =0 since x=0 at y=0  ⇒x=(v_0 /du)y^2  with 0≤y≤(d/2)  at y=(d/2): x=((v_0 d)/(4u))    x=((2v_0 )/u)(y−(y^2 /(2d)))+C_2   at y=(d/2):  ((v_0 d)/(4u))=((2v_0 )/u)[(d/2)−(d/8)]+C_2   C_2 =−((v_0 d)/(2u))  ⇒x=((2v_0 )/u)(y−(y^2 /(2d))−(d/4)) with (d/2)≤y≤d  at y=d: x=((v_0 d)/(2u))    y=ut  x=((v_0 u)/d)t^2  with 0≤t≤(d/(2u))  x=2v_0 (t−((ut^2 )/(2d))−(d/(4u))) with (d/(2u))≤t≤(d/u)
$$\mathrm{u}_{\mathrm{y}} =\mathrm{u} \\ $$$$\mathrm{y}=\mathrm{ut} \\ $$$$\mathrm{u}_{\mathrm{x}} =\frac{\mathrm{2y}}{\mathrm{d}}\mathrm{v}_{\mathrm{0}} =\frac{\mathrm{2v}_{\mathrm{0}} }{\mathrm{d}}\mathrm{y}\:\mathrm{with}\:\mathrm{0}\leqslant\mathrm{y}\leqslant\frac{\mathrm{d}}{\mathrm{2}} \\ $$$$\mathrm{u}_{\mathrm{x}} =\frac{\mathrm{2}\left(\mathrm{d}−\mathrm{y}\right)}{\mathrm{d}}\mathrm{v}_{\mathrm{0}} =\mathrm{2v}_{\mathrm{0}} \left(\mathrm{1}−\frac{\mathrm{y}}{\mathrm{d}}\right)\:\mathrm{with}\:\frac{\mathrm{d}}{\mathrm{2}}\leqslant\mathrm{y}\leqslant\mathrm{d} \\ $$$$ \\ $$$$\mathrm{u}_{\mathrm{x}} =\frac{\mathrm{dx}}{\mathrm{dt}}=\frac{\mathrm{dx}}{\mathrm{dy}}×\frac{\mathrm{dy}}{\mathrm{dt}}=\mathrm{u}\frac{\mathrm{dx}}{\mathrm{dy}} \\ $$$$\Rightarrow\mathrm{u}\frac{\mathrm{dx}}{\mathrm{dy}}=\frac{\mathrm{2v}_{\mathrm{0}} }{\mathrm{d}}\mathrm{y}\:\mathrm{with}\:\mathrm{0}\leqslant\mathrm{y}\leqslant\frac{\mathrm{d}}{\mathrm{2}} \\ $$$$\Rightarrow\mathrm{u}\frac{\mathrm{dx}}{\mathrm{dy}}=\mathrm{2v}_{\mathrm{0}} \left(\mathrm{1}−\frac{\mathrm{y}}{\mathrm{d}}\right)\:\mathrm{with}\:\frac{\mathrm{d}}{\mathrm{2}}\leqslant\mathrm{y}\leqslant\mathrm{d} \\ $$$$\mathrm{x}=\frac{\mathrm{v}_{\mathrm{0}} }{\mathrm{ud}}\mathrm{y}^{\mathrm{2}} +\mathrm{C}_{\mathrm{1}} \\ $$$$\mathrm{C}_{\mathrm{1}} =\mathrm{0}\:\mathrm{since}\:\mathrm{x}=\mathrm{0}\:\mathrm{at}\:\mathrm{y}=\mathrm{0} \\ $$$$\Rightarrow\mathrm{x}=\frac{\mathrm{v}_{\mathrm{0}} }{\mathrm{du}}\mathrm{y}^{\mathrm{2}} \:\mathrm{with}\:\mathrm{0}\leqslant\mathrm{y}\leqslant\frac{\mathrm{d}}{\mathrm{2}} \\ $$$$\mathrm{at}\:\mathrm{y}=\frac{\mathrm{d}}{\mathrm{2}}:\:\mathrm{x}=\frac{\mathrm{v}_{\mathrm{0}} \mathrm{d}}{\mathrm{4u}} \\ $$$$ \\ $$$$\mathrm{x}=\frac{\mathrm{2v}_{\mathrm{0}} }{\mathrm{u}}\left(\mathrm{y}−\frac{\mathrm{y}^{\mathrm{2}} }{\mathrm{2d}}\right)+\mathrm{C}_{\mathrm{2}} \\ $$$$\mathrm{at}\:\mathrm{y}=\frac{\mathrm{d}}{\mathrm{2}}: \\ $$$$\frac{\mathrm{v}_{\mathrm{0}} \mathrm{d}}{\mathrm{4u}}=\frac{\mathrm{2v}_{\mathrm{0}} }{\mathrm{u}}\left[\frac{\mathrm{d}}{\mathrm{2}}−\frac{\mathrm{d}}{\mathrm{8}}\right]+\mathrm{C}_{\mathrm{2}} \\ $$$$\mathrm{C}_{\mathrm{2}} =−\frac{\mathrm{v}_{\mathrm{0}} \mathrm{d}}{\mathrm{2u}} \\ $$$$\Rightarrow\mathrm{x}=\frac{\mathrm{2v}_{\mathrm{0}} }{\mathrm{u}}\left(\mathrm{y}−\frac{\mathrm{y}^{\mathrm{2}} }{\mathrm{2d}}−\frac{\mathrm{d}}{\mathrm{4}}\right)\:\mathrm{with}\:\frac{\mathrm{d}}{\mathrm{2}}\leqslant\mathrm{y}\leqslant\mathrm{d} \\ $$$$\mathrm{at}\:\mathrm{y}=\mathrm{d}:\:\mathrm{x}=\frac{\mathrm{v}_{\mathrm{0}} \mathrm{d}}{\mathrm{2u}} \\ $$$$ \\ $$$$\mathrm{y}=\mathrm{ut} \\ $$$$\mathrm{x}=\frac{\mathrm{v}_{\mathrm{0}} \mathrm{u}}{\mathrm{d}}\mathrm{t}^{\mathrm{2}} \:\mathrm{with}\:\mathrm{0}\leqslant\mathrm{t}\leqslant\frac{\mathrm{d}}{\mathrm{2u}} \\ $$$$\mathrm{x}=\mathrm{2v}_{\mathrm{0}} \left(\mathrm{t}−\frac{\mathrm{ut}^{\mathrm{2}} }{\mathrm{2d}}−\frac{\mathrm{d}}{\mathrm{4u}}\right)\:\mathrm{with}\:\frac{\mathrm{d}}{\mathrm{2u}}\leqslant\mathrm{t}\leqslant\frac{\mathrm{d}}{\mathrm{u}} \\ $$
Commented by mrW1 last updated on 19/Jul/17

Leave a Reply

Your email address will not be published. Required fields are marked *